compassion or forgiveness shown toward someone whom it is within one’s power to punish or harm.
Merriam-Webster Online Dictionary
mercy (noun)1.
a) compassion or forbearance shown especially to an offender or to one subject to one's power , also lenient or compassionate treatment - begged for mercy
b) imprisonment rather than death imposed as penalty for first-degree murder
2.
a) a blessing that is an act of divine favor or compassion
b) a fortunate circumstance - it was a mercy they found her before she froze
3.
compassionate treatment of those in distress - works of mercy among the poor